The refinery, which employs 1,400 people, can refine 220,000 barrels of crude oil per day.

The plant was due to change hands after US firm Valero agreed earlier this year to buy it for 730 million dollars (503 million euros) and pay a further one billion dollars for the stocks of oil, petrol and other products on the site.

Refinery general manager Greg Hanggi said the deaths were "utterly devastating" and sent his sympathies to the victims' families.

"We will ensure that all employees and contractors are fully supported throughout this difficult time," he said.

"We will take every step possible to determine the series of events that led to this tragic incident and ensure that any lessons learnt from it will be integrated into the business and shared with our industry partners."

The last major explosion at an oil installation in Britain took place at the Buncefield depot, north of London, in 2005.

The blast at the depot -- jointly run by Texaco, part of Chevron, and French oil giant Total -- was one of the biggest in peacetime Europe.

It was reportedly heard as far away as France and Belgium and measured 2.4 on the Richter Scale.

One of Britain's costliest industrial disasters, the explosion left 43 people injured and forced 2,000 others to leave their homes. Despite the size of the explosion, no-one was killed.